### Wavelet:

Wavelets are tiny parts of waves.

### Secondary wavefront:

The envelop of all the secondary wavelets in the forward direction is called the secondary wavefront. The backward envelop is not a secondary wavefront.

### Wavefront

A wavefront can be defined as the locus of all the points of the medium which are vibrating in phase and are also displaced at the same time.

If the distance of the source is small the wavefront is spherical. But when the distance is large then any small portion of the wavefront can be considered plane.

### Diffraction grating

Diffraction grating is an extremely useful device. In one of its form it consists of a very large number of narrow slits side by side. The slits are separated by opaque spaces.

### What do you mean by Fresnel and Fraunhofer class of diffraction?

Diffraction phenomena can conveniently divided into two groups.
1. Fresnel Diffraction Phenomena: In the Fresnel class of diffraction, the source or the screen or the both are at finite distances from the aperture or obstacle causing diffraction. In this case, the effect at a specific point on the screen due to the exposed incident wavefront is considered and no modification is made by lenses and mirrors.
2. Fraunhofer Diffraction Phenomena: In the Frauhofer class of diffraction phenomena, the source and the screen on which the pattern is observed are at infinite distances from the aperture or the obstacle causing diffraction. The incoming light is rendered parallel with a lens and the diffracted beam is focused on the screen with another lens.

### Diffraction

The rhythmic variations in intensity and the bending of light around the corners of an obstacle or the encroachment of light into the region of geometrical shadow constitute a class of phenomena known as the diffraction of light.

### Newton’s rings and its formation :

When a Plano-convex lens of long focal length is placed on a plane glass plate, a thin film of air is enclosed between the lower surface of the lens and the upper surface of the plate. The thickness of the air film is very small at the point of contact and gradually increases from the center outwards.

### Fringe width

The distance between any two consecutive bright or dark fringe is known as fringe width. Symbolically the fringe width is β and

### Why two independent source of light can not produce interference?

Two independent sources of light can not produce interference because a large number of difficulties are involved. The two sources may emit light waves of largely different amplitude and wave length and the phase difference between the two may change with time. That’s why interference of light is produced by two coherent sources which are virtual.

### Coherent sources

Two sources are said to be coherent if they emit light waves of same frequency, nearly equal amplitude and are always in phase with each other.

### Interference of light:

Wave emitted from two coherent sources having the same wavelength, equal or nearly equal amplitude when passes through a point in a medium, then superposition takes place.

During superposition, if the two waves reaching the point in the same phase then the intensity will be maximum that is bright. But if the two waves reaching the point in opposite phase then the point will have minimum intensity that is dark. This variation of intensity due to superposition is called interference of light.

The interference is said to be constructive or destructive when the intensity is respectively more or less than that given by one beam alone.

### Huygens principle

To explain the propagation of light through ether, Huygens proposed the following principles for the construction of a wavefront at a subsequent time from the known position of the wavefront at any given instance.

### Stationary waves:

When two simple harmonic waves of the same amplitude, frequency and time period travel in opposite directions in a straight line, the resultant wave obtained is called a stationary or a standing wave. The formation of stationary waves is due to the superposition of the two waves on the particles of the medium.

### Transverse waves:

In transverse wave motion, the particles of the medium vibrate at right angles to the direction of propagation of wave. Example: Light waves are transverse waves.

### Longitudinal wave motion:

In longitudinal wave motion, particles of the medium vibrate along the direction of propagation of the wave. Example: Sound waves are longitudinal waves.

### Wave motion:

Wave motion is a form of disturbance produced in the medium by the repeated periodic motion of the particles of the medium. Here, only the waves travel forward whereas the particles of the medium vibrate about their mean positions.

### Resonance:

When the forced frequency is equal to the natural frequency of vibration of the body, resonance takes place. Resonance is a vibration of large amplitude produced by a relatively small vibration near the same frequency of vibrations as the natural frequency of the resonating system.

### Undamped free vibrations:

When a bob of a simple pendulum (in vacuum) is displaced from its mean position and left, it executes simple harmonic motions. The pendulum will continue to oscillate with the same time period and amplitude for any length of time. In such cases there is no loss of energy by friction or otherwise. In all similar cases, the vibrations will be undamped free vibrations.

### Lissajous’ figure:

When a particle is influenced simultaneously by two simple harmonic motions at right angles to each other, the resultant motion of the particle traces a curve. These curves are called Lissajous’ figure.

### Simple harmonic motion:

The type of motion where the acceleration is directed towards a fixed point (the mean position of rest) and is proportional to the displacement of the vibrating particle is called simple harmonic motion.

### Entropy:

The term ‘Entropy’ is a Greek word and means transformation. Entropy is a physical property or nature of a substance that remain constant in an adiabatic process. It is a quantity that increases with the application of heat in a reversible manner.

If, a process or cycle reject or absorb heat of amount dQ in T temperature, then entropy,

### Carnot’s reversible engine

In 1824, Sadi Carnot conceived a closed reversible cycle consisting two strokes. Since that Carnot’s cycle reversible because it consists two isothermals and two adiabatic, all of which are reversible process.

Carnot’s engine is free from all the defects of practical engine, its efficiency is maximum and it is an ideal heat engine. It has three essential requisites;

### Postulates of the kinetic theory of gases:

Following are the 13 postulates of kinetic theory of gases.

### Isothermal process:

If a system is perfectly conducting to the surroundings and the temperature remains constant throughout the process, then the process is called an isothermal process. In this process, although the temperature remains constant but the heat is variable.

From the given figure, from A to B, pressure decreased and work is done. There should be fall in temperature but the system absorbs heat and maintains a constant temperature.

From B to A, pressure increased and external work is done. There should be rise in temperature, but the system gives up extra heat to the surroundings and the temperature remain constant.

### Second law of thermodynamics

The second law of thermodynamics has Kelvin-Planck statement and Kelvin-Clausius statement. They are stated below.

### First Law of Thermodynamics:

The first law of thermodynamics states that the amount of heat given to a system is equal to the sum of the increase in the internal energy of the system and the external work done.